Light Snow then Heavy Cold

Moisture from a system to our south is coupled with some jet stream support is producing light snow.  The snow will vary in intensity over the next 18-24 hours, but should largely remain light.  In general, 1" to 2" totals will be common, but with surface temperatures running near freezing tonight, road chemicals should keep pavement wet.  However, the follow-up to all of this is the much ballyhooed blast of arctic air which will roar in here tomorrow afternoon.  Morning highs in the upper 20s will yield to the dense, cold-air intrusion resulting in a New Year's arrival time temperature around 13. Blow a 10-20 mph wind into that air and we're talking about wind chills around -10.  The cold air will be stubborn to dislodge here, and over much of the country for the first 2 weeks of January.  I also expect some significant lake-effect snows in Indiana and Michigan, where a Lake-Effect Snow Watch has already been issued starting late tomorrow night.  The lake effect set up may persist well into next week, with astronomical totals certainly possible given the duration of the event.

Mike

WEDNESDAY, DECEMBER 30, 2009

Greetings on this Wednesday.  Here is a link, once again, to the National Weather Service office in Romeoville, Illinois.

This is a list of the top weather stories which they have compiled.

http://www.crh.noaa.gov/news/display_cmsstory.php?wfo=lot&storyid=44368&source=0

Tracy

Harsh Transition to 2010 Coming

Two items on the table: 1) Potent storm system to bring accumulating snows here starting Wednesday PM   2) Major arctic outbreak poised to blast in here on New Years Day. 

A disturbance which will be bringing Dallas a rare 3rd snowfall in December will spread a swath of snowfall into Chicagoland late Wednesday into Thursday.  Early indications are that 2"-4" will be possible into early Thursday.  Then we've got a strong punch of arctic air coming behind that storm.  I will show a chart called the Arctic Oscillation Index on the 4pm news today. The AO is a measure of atmospheric pressure anomalies in the North Polar region.  When it's negative, cold air often attacks much of the northern hemisphere.  Conversely, when it is positive, milder temperatures are often found.  Here's the chart:

http://www.cpc.noaa.gov/products/precip/CWlink/daily_ao_index/ao_index.html

Notice how it was quite low in October, and what did we have here? One of the coldest Octobers on record. Next, look at the graph for November, when temperatures for much of nation and right here in Chicago were unusually warm.  Now, look at how far in the tank it's gone over the past few weeks.  There are many other factors to consider when making a long-range forecast, but as I posted here long ago, there were many reasons not to follow the simple "It's going to be an El Nino winter here" projected by NOAA.  We're one month into meteorological winter, and so far the national and international weather headlines have been dominated by extremes on the cold and snowy side.  Could conditions turn? Absolutely.  But I expect the first week and half of January at least to get off to a gangbuster start.

Mike

The unique snow storm on Saturday

The snow that fell Saturday was quite a surprise, not so much that it snowed but how much snow fell in certain areas. While most of us saw three to six inches, upwards of a foot fell in a small corridor covering much of Kane county into the western and northwestern parts of Dupage county southern McHenry county and portions of western Lake county. For several hours Saturday snow fell at the rate of 1 to 2 inches in these locations. So what went wrong????

First the snow was extremely dry. This is important because with the same amount of moisture we would typically have only seen3 to 5 inches of snow. A common ratio of snow to rain is 10 to 1, in other words 1/2 inch of rain equals 5 inches of snow. The ratio Saturday was 30 to 1 so a half inch of moisture Saturday would translate to 15 inches of snow.

The storm that caused the snow was not your typical storm that tracks out of the southern plains and into Indiana putting us in the heavy snow band on the northwest side of the storm. This was an upper air disturbance that rotated around the big Low that brought us the freezing rain/sleet on Christmas Eve. This disturbance was being pushed by a potent jet stream aloft. Cold air was firmly entrenched when the snow started to develop. As this disturbance pulled warmer air into our area it rose over the cold surface air producing the snow. This "overrunning snow" lasted for several hours before diminishing in the evening.

Snow is always tricky to forecast put when 1/10 of an inch of moisture equals 3 inches of snow it is especially dificult to forecast snow amounts. In any case, apart from a few flurries overnight into early Monday we should see quiet weather for the next few days. Our next chance for snow will come Wednesday night into Thursday but this does not look like a big deal.

Phil

DECEMBER 27, 2009
After larger than expected snowfall totals (especially in our north and west suburbs), it looks like the latest pesky low pressure storm system is not through with us yet. Expect an additional 2-4 inches of snow today. While our entire area will see snow, this time I believe the larger focus will be to the south of I-80 as the national weather service has issued a winter weather advisory (for snow and blowing snow) for areas south of I-80 as well as NW Indiana. Watch out especially for snow blowing and drifting across area roads and highways. This slow moving storm system should be out of here fully by Tuesday with better chances of sunshine. Don't expect the snow to melt anytime soon as more cold weather dominates the 7 day forecast. If you like all this snow and cold then enjoy! Perhaps the kiddies will get a chance to use thier new sleds they got from Santa! Have a great week and happy new year! MARK BISHOP

DECEMBER 26, 2009
Hopefully everyone is having a happy holiday season! After some brief snow showers on Christmas day, it looks like more chances of snow throughout the weekend. The same slow moving storm that brought over a foot of snow and blizzard conditions to Minnesota, the Dakotas, Nebraska, and Kansas will now enter the Midwest with a taste of winter. While we won't see heavy snow, there will be periods of snow that will likely cause plenty of slick spots on area roads.
Along with the snow that could add up to 1-3 inches on Saturday, temps will hardly climb out of the mid-20s. As the low pressure descends slowly eastward over the weekend, expect more chances of light snow on Sunday as well.

First the Ice then the rain

This has been a very unusual storm. The mixture of snow, sleet and freezing rain that fell yesterday afternoon changed to all freezing rain last night before diminishing this morning. The next wave of moisture will hit mainly late today and through tonight. This wave will bring some heavy rains that will wash away most if not all of the snow and ice for Christmas.

 The low responsible for the nasty weather will stall out over Iowa and actually intensify. Cold air will wrap all the way around the low so that temperatures will fall on Christmas day even though we will have a strong wind out of the south. Snow will return tomorrow night and continue sporadically throughout the weekend. By the time Sunday night rolls around we should see 1 to 3 inches of fresh snow on the ground.

 With the slow movement of the low snow will be exceedingly heavy with 1 to 2 feet of snow expected from Nebraska to Minnesota. Accumulating snow will fall all the way down into Dallas where a couple inches will fall this afternoon.

Next week looks cold but dry as we will finally see a less active pattern develop.

Wishing everyone out there Happy Holidays. Stay safe and enjoy the season.

  Phil

Ice Storm Warning

Our feared severity of the icing risk has now been echoed by the National Weather Service.  At about 8:45 tonight, they issued an Ice Storm Warning for most of the Chicago area.  It's worth pointing out that the warning does not include Cook County.  While areas close to Lake Michigan MAY miss out on significant icing, I'd be pretty concerned about folks in the northern and western parts of Cook County.  Hey, the ice was bad enough on the top of Willis Tower to knock the weather service's own radio station off the air tonight. Besides the obvious risks on the road, this ice is going to weigh down a lot of tree limbs and power lines.  It would not surprise me if thanks to 30+mph winds, many folks lose their power overnight into the morning.  Please be prepared for that possibility.  I can't see us getting much above freezing until tomorrow afternoon in all honesty, so the ice may hang on for quite some time making matters worse. Then we'll have to contend with the heavy rain and flooding risk tomorrow night into Christmas.  Joy Joy Joy.  Drive and walk carefully and be ready for several days of generally unpleasant weather.

Mike

Holiday Hullaballoo in Progress

Just about everything that can fall from a cloud other than hail is likely to bombard the Chicago area from now well into the weekend as a lethargic storm system slides slowly up the Mississippi Rver Valley.  The biggest problems will be due to a thick accretion of ice as rain, sleet and even some snow fall onto objects near the ground that are below freezing.  As of this writing, a winter weather advisory is in effect for the veiwing area, set to expire at 3pm for all but the counties next to Wisconsin.  I find this hard to accept.  There is consensus that the icing will last much longer than 3pm throughout the area.  The notable exception may be within a few miles of the lake, where the mercury may nudge above freezing earlier.  Here's the point: icing will be likely right into tomorrow. The area-wide changeover to plain liquid rain is likely around mid-day tomorrow, although some of you south of Chicago will get into the warmer air sooner.  Drive carefully.  Call the airline as cancellations have already been reported.  Exercise caution walking around as sidewalks, porches and parking lots will be slicker than snot on teflon.

Mike

Winter Storm Coming

Today's snowfall lingers on, with totals in the 1" to 4" range quite common.  The "Bigger" storm in terms of impact is still due here Wednesday into Friday.  Consequently there is Winter Storm Watch posted for Chicagoland from tomorrow afternoon through Thursday morning, when the greatest risk of significant icing and snow exists.  A large storm system is gathering over the Rockies and will be diving into the Plains, taking a huge gulp of Gulf moisture as it makes the turn northeastward over the holiday.  The thermal profile of the atmosphere suggests precipitation will likely break out tomorrow as snow, then mix with some sleet during the day. A couple of inches of slop may accumulate before the most potentially dangerous portion of the storm takes place.  As the layer of sub-freezing air shrinks to just a few tens of feet above the ground, the air aloft will be warming, changing the snow and sleet over to liquid.  As this rain falls onto surfaces that are at or below freezing on the ground, it will create a glaze of ice.  This ice will makes roads highly treacherous, especially on secondary routes and bridges. It appears this is most likely tomorrow night into Christmas Eve morning.  After that, temperatures are expected to warm well above freezing, changing everything over to just plain old cold rain.  Trouble is, if trees and power lines get coated with 1/4" of ice, gusty winds of 25+ mph could create power outages knocking out a lot of those holiday lights.  While the exact timing and location of ice vs. snow vs. rain is uncertain, there's no argument that this storm will have a major impact on holiday travel plans around and through Chicago.  Jerry, Phil and I will have the very latest on the storm so stay tuned!

Mike